The present invention relates to a reflective optical element for the extreme ultraviolet and soft X-ray wavelength region having a reflective surface with a multilayer system. Moreover, the present invention relates to an optical system for EUV lithography with such a reflective optical element.
For the lithography of semiconductor components, in EUV lithography devices use is made of reflective optical elements for the extreme ultraviolet (EUV) and soft X-ray (SX) wavelength region (for example wavelengths of between approximately 5 nm and 20 nm) such as, for example, photomasks or mirrors on the basis of multilayer systems. Since EUV lithography devices generally have a plurality of reflective optical elements, these elements must have as high a reflectivity as possible in order to ensure a sufficiently high total reflectivity. The reflectivity and the service life of the reflective optical elements can be reduced by contamination of the optically used reflective surface of the reflective optical elements. This contamination results from the shortwave irradiation together with residual gases in the operating atmosphere. Since a plurality of reflective optical elements are usually arranged one behind another in an EUV lithography device, even relatively small amounts of contaminations on each individual reflective optical element affect the total reflectivity to a relatively great extent.
Contamination can occur, for example, because of moisture residues. In this case, water molecules are dissociated by the EUV and/or SX radiation, and the resulting free oxygen radicals oxidize the optically active surfaces of the reflective optical elements. A further source of contamination is polymers. These can, for example, originate from the vacuum pumps used in EUV lithography devices or from residues of photoresists that are used on the semiconductor substrates to be patterned, and can lead, under the influence of the operating radiation, to carbon contaminations on the reflective optical elements. Whereas oxidative contaminations are generally irreversible, it is possible, in particular, to remove carbon contaminations through, inter alia, treatment with reactive hydrogen by having the reactive hydrogen react with the carbon containing residues to form volatile compounds. Reactive hydrogen can be hydrogen radicals or else ionized hydrogen atoms or molecules.
However, it has been observed that under the influence of reactive hydrogen that is used for cleaning, or because of the interaction of the EUV and/or SX radiation with hydrogen present in the residual atmosphere, it is possible that individual layers, in particular those right at the surface of the multilayer system, can become detached. It is thought that the hydrogen, for example in silicon layers, reacts to form silane compounds.